1. Field of the Invention.
The present invention relates to electronic amplifier circuits. In particular, the present invention is an audio power amplifier which can be directly coupled to a load.
2. Description of the Prior Art.
Power amplifiers are well known and disclosed, for example, in U.S. Pat. Nos. 3,191,126 granted to Fowler in June 1965, 3,042,875 granted to Higginbothan in July 1962, and 4,229,706 granted to Bongiorno in October 1980. Desirable characteristics of audio power amplifiers include a high damping factor, wide frequency response, high transient response, and low overall distortion. The tradeoff in achieving these characteristics is, of course, overall cost of the amplifier.
While the vast majority of audio power amplifiers on the market today are transistorized, extremely high performance audio power amplifiers often use vacuum tubes as output devices. Vacuum tube power amplifiers, however, typically require a matching transformer to match the high impedance of the tube circuits to the low impedance of the loudspeaker load. The matching transformer also prevents the high voltages found in conventional tube circuits from damaging the loudspeaker.
One vacuum electron tube audio power amplifier which does not use a matching transformer at its output is known as the Futterman Amp. This particular amplifier design uses a well known "totem pole" output like many solid-state designs, and shares some of the same weaknesses. Some versions of this amplifier also include coupling capacitors to block harmful DC voltages. The output coupling capacitors, however, adversely affect the slewing response of the amplifier, and also introduce non-linearities into its phase response. As a result, the output coupling capacitors have many of the same limitations as the matching transformers.
Totem pole amplifiers, which comprise the majority of solid-state designs, as well as direct-coupled tube designs, have inferior distortion caracteristics. These distortion characteristics, when coupled to the odd-ordered harmonic distortions of solid-state devices, make totem pole amplifier configurations less than desirable.
It is evident that there is a continuing need for improved audio power amplifiers. An audio power amplifier design which permits the use of vacuum tube output devices which can be directly coupled to a loudspeaker load would be especially desirable. This amplifier must be capable of producing high output power levels over a wide bandwidth with low distortion. Of course, the design must be simple, to allow easy implementation.